Generally, a mobile communication system includes multiple base stations, each having a wide communication area, so as to implement wide area communication. However, since the intensity of radio waves from each base station degrades under ground or inside a building, communication may be difficult in such conditions. In order to solve this problem, a secondary base station called a “pico base station” or “femto base station” having a small communication area is used.
FIG. 9 is a diagram showing the structure of an ordinary communication system having a femto base station.
The communication system includes a peripheral base station 910, a terminal 920, and a femto base station 930.
The service frequency of the peripheral base station 910 is α, and the service frequency of the femto base station 930 is γ.
FIG. 10 is a sequence chart showing the operation of the ordinary communication system.
First, the peripheral base station 910 generates peripheral base station announcement information which includes adjacent cell information (see step S91). The adjacent cell information includes information about the femto base station 930.
When generating the peripheral base station announcement information, the peripheral base station 910 sends the generated peripheral base station announcement information by using a frequency α at a regular transmission timing (see step S92).
When the peripheral base station 910 sends the peripheral base station announcement information, the terminal 920 receives the peripheral base station announcement information from the peripheral base station 910 by using the frequency α (see step S93).
In the above process, the terminal 920 belongs to a communication area covered by the peripheral base station 910.
When receiving the peripheral base station announcement information, the terminal 920 performs an adjacent call search where target adjacent cells include the femto base station 930 (see step S94).
The femto base station 930 sends femto base station announcement information by using a frequency γ at a regular transmission timing (see step S95).
When the terminal 920 performs the adjacent call search in step S94, the terminal 920 receives the femto base station announcement information from the femto base station 930 by using the frequency γ (see step S96).
In this process, if the terminal 920 receives stronger radio waves from the femto base station 930 than those from the peripheral base station 910, the terminal 920 changes the communication area to which it belongs, from that of the peripheral base station 910 to that of the femto base station 930, so that communication via the femto base station 930 can be performed.
If the terminal 920 receives stronger radio waves from the peripheral base station 910 than those from the femto base station 930, the terminal 920 does not change the communication area to which it belongs, and performs communication via the peripheral base station 910.
Patent Document 1 discloses a technique for updating adjacent cell information included in the peripheral base station information which is sent by the peripheral base station 910.